92 resultados para linear stability analysis
Resumo:
In this paper, effect of strain gradient on adiabatic shear instability in particle reinforced metal matrix composites is investigated by making use of the strain gradient dependent constitutive equation developed by Dai et al. [9] and the linear perturbation analysis presented by Bai [10]. The results have shown that the onset of adiabatic shear instability in metal matrix composites reinforced with small particles is more prone to occur than in the composites reinforced with large particles. This means that the strain gradient provides a strong deriving force for onset of adiabatic shear instability in metal matrix composites.
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Thermocapillary instabilities on floating half zone convection in microgravity environment were investigated by linear instability analysis method. The critical Marangoni numbers were obtained and compared with the experimental ones. The influences of the liquid bridge volume and the aspect ratio on the critical Marangoni number were analyzed. It is found that the liquid bridge volume and the aspect ratio have great influence on the critical Marangoni number. There was a gap region where the oscillatory convection will not be observed in present analyses and in experiments in the curve of the critical Marangoni number vs the liquid bridge volume for the case of large Prandtl number and small aspect ratio.
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The dynamic behaviour for nanoscale electrostatic actuators is studied. A two Parameter mass-spring model is shown to exhibit a bifurcation from the case excluding an equilibrium point to the case including two equilibrium points as the geometrical dimensions of the device are altered. Stability analysis shows that one is a stable Hopf bifurcation point and the other is an unstable saddle point. In addition, we plot the diagram phases, which have periodic orbits around the Hopf point and a homoclinic orbit passing though the unstable saddle point.
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This paper provides an overview of ongoing studies in the area of thermocapillary convection driven by a surface tension gradient parallel to the free surface in a floating zone. Here, research interests are focused around the onset of oscillatory thermocapillary convection, also known as the transition from quasisteady convection to oscillatory convection. The onset of oscillation depends on a set of critical parameters, and the margin relationship can be represented by a complex function of the critical parameters. The experimental results indicate that the velocity deviation of an oscillatory flow has the same order of magnitude as that of an average flow, and the deviations of other quantities, such as temperature and free surface radii fluctuations, are much smaller when compared with their normal counterparts. Therefore, the onset of oscillation should be a result of the dynamic process in a fluid, and the problem is a strongly nonlinear one. In the past few decades, several theoretical models have been introduced to tackle the problem using analytical methods, linear instability analysis methods, energy instability methods, and unsteady 3D numerical methods. The last of the above mentioned methods is known to be the most suitable for a thorough analysis of strong nonlinear processes, which generally leads to a better comparison with the experimental results. The transition from oscillatory thermocapillary convection to turbulence falls under the studies of chaotic behavior in a new system, which opens a fascinating new frontier in nonlinear science, a hot research area drawing many recent works. This paper reviews theoretical models and analysis, and also experimental research, on thermocapillary connection in floating zones. It cites 93 references.
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Direct numerical simulation of transition How over a blunt cone with a freestream Mach number of 6, Reynolds number of 10,000 based on the nose radius, and a 1-deg angle of attack is performed by using a seventh-order weighted essentially nonoscillatory scheme for the convection terms of the Navier-Stokes equations, together with an eighth-order central finite difference scheme for the viscous terms. The wall blow-and-suction perturbations, including random perturbation and multifrequency perturbation, are used to trigger the transition. The maximum amplitude of the wall-normal velocity disturbance is set to 1% of the freestream velocity. The obtained transition locations on the cone surface agree well with each other far both cases. Transition onset is located at about 500 times the nose radius in the leeward section and 750 times the nose radius in the windward section. The frequency spectrum of velocity and pressure fluctuations at different streamwise locations are analyzed and compared with the linear stability theory. The second-mode disturbance wave is deemed to be the dominating disturbance because the growth rate of the second mode is much higher than the first mode. The reason why transition in the leeward section occurs earlier than that in the windward section is analyzed. It is not because of higher local growth rate of disturbance waves in the leeward section, but because the growth start location of the dominating second-mode wave in the leeward section is much earlier than that in the windward section.
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Numerical simulations were conducted to study thermocapillary flows in short half-zone liquid bridges of molten tin with Prandtl number Pr = 0.009, under ramped temperature difference. The spatio-temporal structures in the thermocapillary flows in short half-zone liquid bridges with aspect ratios As = 0.6, 0.8, and 1.0 were investigated. The first critical Marangoni numbers were compared with those predicted by linear stability analyses (LSA). The second critical Marangoni numbers for As = 0.6 and 0.8 were found to be larger than that for As = 1.0. The time evolutions of the thermocapillary flows exhibited unusual features such as a change in the azimuthal wave number during the three-dimensional stationary (non-oscillating) flow regime, a change in the oscillation mode during the three-dimensional oscillatory flow regime, and the decreasing and then increasing of amplitudes in a single oscillation mode. The effects of the ramping rate of the temperature difference on the flow modes and critical conditions were studied as well. In this paper, the experimental observability of the critical conditions was also discussed. (C) 2008 Elsevier Inc. All rights reserved.
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The thermovibrational instability of Rayleigh-Marangoni-Benard convection in a two-layer system under the high-frequency vibration has been investigated by linear instability analysis in the present paper. General equations for the description of the convective flow and within this framework, the generalized Boussinesq approximation are formulated. These equations are dealt with using the averaging method. The theoretical analysis results show that the high-frequency thermovibrations can change the Marangoni-Benard convection instabilities as well as the oscillatory gaps of the Rayleigh-Marangoni-Benard convection in two-layer liquid systems. It is found that vertical high-frequency vibrations can delay convective instability of this system, and damp the convective flow down. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.
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研究了超音速钝锥绕流的稳定性和转捩点预报的数值计算方法,首先采用Euler方程求解钝锥绕流基本流场,用所得到的物面压力分布作为粘性边界层的外缘压力分布,给出了基本流场的初值;然后应用反迭代法与边界层渐近匹配的方法求解了钝锥边界层的稳定性方程,得到了钝锥边界层转捩数据,该方法可提高计算精度,节约计算时间。
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为解决网络延时对网络化控制系统性能的影响,从控制的角度提出基于系统模型的补偿器设计方案以解决网络延时问题.通过对广义预测控制算法GPC状态空间形式的推导,设计具有多步预测功能的网络控制器,实现前向通道的延时补偿;构造具有延时补偿功能的状态观测器以补偿反馈通道延时.分析了使用上述延时补偿策略所构成的闭环网络控制系统的稳定性.通过对不同网络延时补偿的仿真实验,证实了该补偿算法能有效改善控制系统性能并保持系统的稳定.
Resumo:
圆截面光滑直管内充分发展的两流体同心环状流的线性稳定性研究不仅具有重要的学术意义,而且在预测两相流型转换方面有着重要应用。本文评述了该流动构型的线性稳定性研究进展,着重分析了该流动构型的失稳机制及其与两相流型转换间的关系,并针对微重力气-液两相流地面模拟实验问题,探讨了今后需要着重研究的若干方面。
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针对二维和三维边坡,通过对极限平衡分析结果所依赖的各种因素的分析,探讨了确定安全系数的必要和充分条件.建立了针对不同假设条件的安全系数的解析表达式.这些结果可以使得对极限平衡方法结果的上、下限的估计成为可能.
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The Rayleigh-Marangoni-Benard convective instability (R-M-B instability) and flow patterns in the two-layer system of silicon oil 10cSt and Fluorinert FC70 liquids are studied theoretically and experimentally. Both linear instability analysis and 2D numerical simulation (A=L/H=10) were performed to study the influence of thermocapillary force on the convective instability of the two-layer system. Time-dependent oscillations arising at the onset of convection were investigated in a larger various range of two-layer depth ratios (Hr=H1/H2) from 0.2 to 5.0 for different total depth less than 12mm. Our results are different from the previous study on the Rayleig-B閚ard instability and show the strong effects of thermocapillary force at the interface on the time-dependent oscillations at the onset of instability convection. Primary experimental results of the critical instability parameters and the convective structure in the R-M-B convection have been obtained by using the digital particle image velocimetry (DPIV) system, and a good agreement in comparison with the results of numerical simulation was obtained.
Resumo:
A large diameter cylinder inserted in soils is a new type of engineering structures used in offshore and port engineering. The mechanism of its bearing capacity and the analysis of its stability are important to its design and applications. In this paper, the finite element method is used to analyze the reacting forces of the soft soil foundation on the structure under the wave action. A simplified method is proposed, based on the plastic limit method, for the safety and stability analysis. Our analysis shows that the assumptions made in this paper and the mechanism used are reasonable, and the results obtained are appropriate. The calculation method is very efficient and can be used to evaluate main parameters of the structure in its preliminary designs.
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Table of Contents
1 | Introduction | 1 |
1.1 | What is an Adiabatic Shear Band? | 1 |
1.2 | The Importance of Adiabatic Shear Bands | 6 |
1.3 | Where Adiabatic Shear Bands Occur | 10 |
1.4 | Historical Aspects of Shear Bands | 11 |
1.5 | Adiabatic Shear Bands and Fracture Maps | 14 |
1.6 | Scope of the Book | 20 |
2 | Characteristic Aspects of Adiabatic Shear Bands | 24 |
2.1 | General Features | 24 |
2.2 | Deformed Bands | 27 |
2.3 | Transformed Bands | 28 |
2.4 | Variables Relevant to Adiabatic Shear Banding | 35 |
2.5 | Adiabatic Shear Bands in Non-Metals | 44 |
3 | Fracture and Damage Related to Adiabatic Shear Bands | 54 |
3.1 | Adiabatic Shear Band Induced Fracture | 54 |
3.2 | Microscopic Damage in Adiabatic Shear Bands | 57 |
3.3 | Metallurgical Implications | 69 |
3.4 | Effects of Stress State | 73 |
4 | Testing Methods | 76 |
4.1 | General Requirements and Remarks | 76 |
4.2 | Dynamic Torsion Tests | 80 |
4.3 | Dynamic Compression Tests | 91 |
4.4 | Contained Cylinder Tests | 95 |
4.5 | Transient Measurements | 98 |
5 | Constitutive Equations | 104 |
5.1 | Effect of Strain Rate on Stress-Strain Behaviour | 104 |
5.2 | Strain-Rate History Effects | 110 |
5.3 | Effect of Temperature on Stress-Strain Behaviour | 114 |
5.4 | Constitutive Equations for Non-Metals | 124 |
6 | Occurrence of Adiabatic Shear Bands | 125 |
6.1 | Empirical Criteria | 125 |
6.2 | One-Dimensional Equations and Linear Instability Analysis | 134 |
6.3 | Localization Analysis | 140 |
6.4 | Experimental Verification | 146 |
7 | Formation and Evolution of Shear Bands | 155 |
7.1 | Post-Instability Phenomena | 156 |
7.2 | Scaling and Approximations | 162 |
7.3 | Wave Trapping and Viscous Dissipation | 167 |
7.4 | The Intermediate Stage and the Formation of Adiabatic Shear Bands | 171 |
7.5 | Late Stage Behaviour and Post-Mortem Morphology | 179 |
7.6 | Adiabatic Shear Bands in Multi-Dimensional Stress States | 187 |
8 | Numerical Studies of Adiabatic Shear Bands | 194 |
8.1 | Objects, Problems and Techniques Involved in Numerical Simulations | 194 |
8.2 | One-Dimensional Simulation of Adiabatic Shear Banding | 199 |
8.3 | Simulation with Adaptive Finite Element Methods | 213 |
8.4 | Adiabatic Shear Bands in the Plane Strain Stress State | 218 |
9 | Selected Topics in Impact Dynamics | 229 |
9.1 | Planar Impact | 230 |
9.2 | Fragmentation | 237 |
9.3 | Penetration | 244 |
9.4 | Erosion | 255 |
9.5 | Ignition of Explosives | 261 |
9.6 | Explosive Welding | 268 |
10 | Selected Topics in Metalworking | 273 |
10.1 | Classification of Processes | 273 |
10.2 | Upsetting | 276 |
10.3 | Metalcutting | 286 |
10.4 | Blanking | 293 |
Appendices | 297 | |
A | Quick Reference | 298 |
B | Specific Heat and Thermal Conductivity | 301 |
C | Thermal Softening and Related Temperature Dependence | 312 |
D | Materials Showing Adiabatic Shear Bands | 335 |
E | Specification of Selected Materials Showing Adiabatic Shear Bands | 341 |
F | Conversion Factors | 357 |
References | 358 | |
Author Index | 369 | |
Subject Index | 375 |
Resumo:
目录
- 访罗湖 忆当年[李佩]
- 怀念同窗益友郭永怀教授[钱伟长]
- 郭永怀追求科学、为国献身的一生[洪友士]
- 深深怀念为核盾牌献身的著名科学家--郭永怀先生 中国工程物理研究院
- 深切怀念郭永怀教授的奠基性贡献 中国空气动力研究与发展中心
- 胸怀长江长城 心系国家安危--纪念我国核防护工程奠基人郭永怀先生[周丰峻]
- 缅怀我们的首任系主任--郭永怀教授[辛厚文 马兴孝 俞书勤 何天敬]
- 科学和技术结合的典范--纪念郭永怀先生诞辰九十周年[郑哲敏 李家春]
- 怀念与感激[俞鸿儒]
- 继承先师遗愿,谈我所冲击动力学的发展--纪念郭永怀副院长诞辰九十周年[陈裕泽]
- 英名长存--纪念郭永怀副院长诞辰九十周年[沈中毅]
- 郭永怀与我国导弹弹头再入气动物理研究[魏叔如]
- 科研工作引路人--纪念郭永怀九十诞辰[胡在军]
- 科技楷模,引路良师--怀念敬爱的郭永怀副院长[朱竟洪 郑百瑛]
- 七律 永怀永怀郭老师[董务民]
- 留得春晖映核魂--怀念我们的好领导好老师郭永怀同志[孙德纶 张克才]
- 难以忘怀--忆郭永怀副院长事迹点滴[孙天雄]
- 研究工作与工程技术工作之间的关系[谈庆明]
- 忆敬爱的郭永怀老师[张兆顺 呼和敖德]
- 根深叶茂 厚积薄发--追忆恩师郭永怀先生教诲我打好基础[严宗毅]
- 力学前辈,科德风范[徐友钜]
- 领导的楷模 学者的典范 青年的导师--怀念郭永怀副院长[于长勤]
- 怀念郭永怀副院长[陈裕泽]
- 关于郭永怀事迹的回顾[张长富]
- 严谨的导师,可亲的长者--忆著名的空气动力学家郭永怀先生[吴兰春]
- 严谨细致 实事求是--纪念郭永怀九十诞辰[李启廉]
- 纪念核武器环境试验技术的奠基人--郭永怀副院长诞辰九十周年[李荣林]
- 郭老精神激励着材料科学研究的不断深入[周维宣 谭云]
- 郭永怀副院长的关怀与我所有机材料研究的发展[陈晓丽]
- 忆与郭永怀副院长的一次谈话[孙德纶]
- 缅怀郭永怀院士[陈家镛]
- 记郭永怀先生二三事[屠善澄]
- Remembering Yunghuai Kuo[C.C.Lin]
- Recall with love and respect[William R.Sears]
- Y.H.Kuo:An appreciation[Frank E.Marble]
- A commemorative tribute to Professor Guo Yonghuai[T.Y.Wu]
- Y.H.Kuo:A great scientist,revered teacher and good friend[Alfred Ritter]
- 氢氧燃烧及爆轰驱动激波管[俞鸿儒]
- 颗粒材料中致密波结构研究[孙锦山 朱建士 贾祥瑞]
- 网格与高精度差分计算问题[张涵信 呙超 宗文刚]
- 植被层湍流的大涡模拟[李家春 谢正桐]
- 半浮区液桥热毛细振荡流[唐泽眉 阿燕 胡文瑞]
- 再入湍流尾迹及其对雷达散射的影响研究[牛家玉 于明]
- 超声射流中CS2分子态分辨转动弛豫研究[陈从香 刘世林 戴静华 张志萍 马兴孝]
- 37mm冲压加速器实验和计算[柳森 简和祥 白智勇 平新红 部绍清]
- 化学体系中噪声诱导的时空有序结构和随机共振[辛厚文 侯中怀 杨灵法]
- 根据守恒律计算热流和摩阻的有限元提法[童秉纲 段占元]
- 高阶精度线性耗散紧致格式的渐近稳定性[邓小刚 毛枚良]
- 离散流体力学:理论和数值方法[高智]
- 2号复合离心机自动控制系统研制[王磊 林明 冯晓军]
- 海沧大桥气动弹性特性风洞试验研究[李明水 陈忻 张大康 王卫华]
- 内爆炸载荷下圆管变形、损伤和破坏规律的研究[李永池 李大红 魏志刚 孙宇新]
- 两种凝聚炸药的静态断裂性能实验研究[罗景润 韦日演 马丽莲]
- 圆柱体侵彻薄靶极限击穿速度估算探讨[吴应白 唐平]
- 动力学修改方法在夹具设计中的应用研究[蒲怀强 唐定勇]
- Numerical simulation of non-linear stability of two-dimensional supersonic boundary layer[Shen Qing Yuan Xiangjiang]